The IEEE 802.3ba standard on 40/100 Gigabit Ethernet (40/100 GbE) was ratified in June 2010 as a response to the ever-increasing demands for higher capacity transmission over optical fiber links. Discussions over the next upgrade for Ethernet have already started. While some argue that 1 Terabit Ethernet (TbE) should be standardized next to meet the projected demand, some advocate taking a more conservative step with an intermediary upgrade to 400 GbE. Regardless of such projections, all agree that the enormous growth in Ethernet traffic will render higher capacity transmission inevitable, and 100 GbE is just another station but not the final destination in the evolution of Ethernet.
As the operating symbol rates increase, the deteriorating effects of fiber nonlinearities and polarization-mode dispersion (PMD) reach levels that inhibit reliable communication over the optical fiber network. Thus solutions for 100 GbE and beyond need to attain ultra-high transmission speeds in terms of aggregate bit rates while keeping the operating symbol rates low to facilitate nonlinearity and PMD management. A promising solution employing coded modulation using low-density parity-check (LDPC) codes as component codes has already been discussed in our previous works [1], [2]. The underlying idea is to use spectrally-efficient modulation formats at low symbol rates along with strong LDPC codes for forward error correction (FEC) in order to accomplish reliable communication at high aggregate bit rates.
In addition to spectral efficiency, power efficiency of a modulation format plays an important role in communication system design. Coherent optical communication physically allows using four dimensions for modulation rather than only two dimensions over which conventional modulation formats, e.g., quadrature amplitude modulation, are defined. As a result, one can exploit this four-dimensional (4D) signal space to set up more power-efficient signal constellations than one could do using the 2D signal space—by increasing the Euclidean distance between constellation points for a given average signal power.